Infrared imagers convert infrared radiation into electrical signals, much as television cameras convert a light scene into television signals which may be processed and viewed. Infrared imagers are widely used for nighttime viewing, tracking of hot objects, and verifying building insulation. They are also used in medicine for detecting thermal abnormalities, as in mammography. For many of these applications, interest is directed to a particular infrared spectrum.
Infrared imagers tend to be relatively nonselective as to the nature of the photon energy to which they respond. For example, imager elements (converters of infrared radiation into electrical signals) which are sensitive to infrared radiation are likely to also be responsive to visible light. For this reason, it may be desirable to use a spectrum-limiting filter placed to intercept the undesired portion of the radiation spectrum reaching the imager. The filters for limiting the spectrum of the radiation reaching the imager element are transparent within the selected spectrum. Outside the selected spectrum, the filters may have a high emissivity or capability of radiation, akin to that of a black-body radiator.
In order to maximize the sensitivity of the infrared imager, it is necessary to reduce the stray radiation falling on the photosensitive surface of the imager element. In particular, infrared radiation attributable to thermal effects in structures adjacent to the imager element must be controlled. This control may be achieved by reducing the emissivity of the material of which the structures are constructed, or by reducing the temperature, or both. In general, it is undesirable to use low-emissivity materials, such as polished metals, near the imager, since their low emissivity is offset by the effects of relatively great reflectivity, which causes any stray radiation which happens to be present to reflect from the low emissivity surface rather than being absorbed. Therefore, the structures surrounding the imager, and the imager element itself, are desirably maintained at a low temperature. It is common to house the imager element and the surrounding structures in a "cold box," which is maintained at a cold or low temperature. The low temperature may be provided by a Dewar vessel of liquified gas such as liquid air or liquid nitrogen. For less demanding applications at the current state of the art, one or more electrically energized Peltier thermocouples may be used to cool the cold box. Such thermocouples are electrically actuated heat pumps having a cold surface and a hot surface, and may be cascaded to decrease the temperature of the cold surface which is in contact with the cold box or with the imager and surrounding structures.
A lens assembly is used in infrared imagers to focus radiation from the object being imaged onto the photosensitive surface of the imager element. The lens assembly includes one or more lens elements, and a mounting structure therefor. The lens assembly mounting, and other portions of the lens assembly are part of the structure adjacent the imager which may contribute to the stray radiation which generates noise in the imager signal and which in turn reduces the useful sensitivity of the imager.
It is desirable to enhance the sensitivity of the imager. It is further desirable to be able to conveniently change the field of view of an infrared imager, or to conveniently select the infrared spectrum to which it responds, without disassembly of the cold box and its associated structures.